Twice a month, at the new moon and the full moon, the sun and the moon pull in line. That makes the biggest tides — the “spring” tides: the high water comes up further, the low water goes out further, and the currents run hardest. These are the days the water moves most.
In between, at the half-moons (the quarters), the sun and the moon pull at angles to each other. The tides are at their smallest — the “neap” tides: less range, gentler currents.
The biggest tides actually run a day or two after the new or full moon, not on the exact day — so treat these as the strong stretch, not a single date.